RESEARCH PAPERS FACULTY OF MATERIALS SCIENCE AND TECHNOLOGY IN TRNAVA, SLOVAK UNIVERSITY OF TECHNOLOGY IN BRATISLAVA, 2016 Volume 24, Number 38

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1 RESEARCH PAPERS FACULTY OF MATERIALS SCIENCE AND TECHNOLOGY IN TRNAVA SLOVAK UNIVERSITY OF TECHNOLOGY IN BRATISLAVA 2016 Volume 24, Number 38 THE STABILITY OF WEDM Vladimír ŠIMNA SLOVAK UNIVERSITY OF TECHNOLOGY IN BRATISLAVA, FACULTY OF MATERIALS SCIENCE AND TECHNOLOGY IN TRNAVA, INSTITUTE OF PRODUCTION TECHNOLOGIES, ULICA JÁNA BOTTU 2781/25, TRNAVA, SLOVAK REPUBLIC Abstract This paper is focused on the effect of generator s parameters on the stability of WEDM process that is expressed by the length of time of cutting until the wire electrode breaks. This paper provides an overview of the current research articles dealing with the causes of the wire electrode s rupture and the options of control and regulation of the process. The aim was to clarify the impact of input parameters to the cutting process, and to find those parameters making the process as fast as possible and stable. The experiment was performed on the machine of Agie Charmilles Robofil 310. As input factors were selected pulse width, time between two pulses, servo voltage, pulse current, short pulse time and frequency. Due to the fact that it is necessary to assess the impact of several input parameters, Taguchi experimental design was selected. L27 matrix was used. The Analysis of Variance (ANOVA) was used for evaluation. Key words stability, WEDM, Material Removal Rate, ANOVA, Taguchi INTRODUCTION Material Removal Rate (MRR) is generally expressed as a proportion of the volume of removed material and time for which it has been taken. Sometimes it is expressed as a weight of removed material for a time. In the case of EDM wire cutting, the following formula is the most commonly used for the calculation (1): where: vc -cutting speed [mm.min -1 ], B -width of the cut [mm], H - height of the cut. MRR = vc.b.h [mm 3.min 1 ], [1] 67

2 The following basic parameters of wire EDM can be considered: pulse on time, pulse off time, voltage, peak current, discharge current, flushing pressure of dielectric fluid, wire feed rate and wire tension. Each of these parameters (except wire tension and wire feed rate) has huge influence on the cutting process, which is explained in detail in (2). Research of the relationship between the cutting speed and input parameters has been described in (3). The cemented carbide has been machined. It has been found that the increase of pulse duration leads to a higher cutting speed, greater duration of the pulse off time, the more cutting speed decreases and the voltage affects the cutting speed inversely, and, with increasing discharge current, cutting speed increases. It was found that the discharge current and the discharge pulse duration have a greater impact than other observed parameters on the MRR, surface roughness and wear wire (4). In (5) it has been proved that discharge frequency, voltage, pulse duration and wire feed rate has the greatest influence on the MRR. S. R. NITHIN ARAVIND et al. (2012) (6) present the experimental study to select best suitable value of voltage, current, speed, pulse on/off time in order to get maximum metal removal rate (MRR) and minimum surface roughness (SR). METHODS AND MATERIALS There are many parameters that enter into the WEDM process. The aim of the experiment was to determine the effect of the process input parameters to break the wire electrode. In this case, it was necessary to evaluate the importance of each parameter in the cutting process. For this experiment, Taguchi experimental design was used. Six parameters were used at three levels, as can be seen in Table 1. It means L27 orthogonal array was used. Table 1 Used parameters Parameter Label Unit Factor Level Level 1 Level 2 Level 3 Pulse width A µs Time between two pulses Servo reference mean voltage B Aj µs V Injection pressure INJ Bar Wire tension Wb N Frequency FF % Other parameters (Voltage, Pulse current, Short pulse time, Feed rate, Wire feed speed) were the constants, and their values can be seen in Table 2. 68

3 Table 2 Constant parameters Parameter Label Unit Value Voltage V V -80 Pulse current IAL A 8 Short pulse time TAC µs 0.5 Linear feed rate S mm/min 5 Wire feed speed Ws mm/min 15 As the workpiece material, high speed molybdenum-wolfram-vanadium steel (HS6-5-2C) was used. Steel was nitrided in NH3. The width of the workpiece was 50 mm. The experiment was carried out on the Charmilles ROBOFIL 310 machine, which is a CNC machine for wire EDM. The diameter of the CuZn37 wire was 0.25 mm. Wire tensile strength Rm was 980 N.mm -2. As dielectricum deionized water was used. Conductivity of the dielectricum was 5 S. RESULTS Table 3 shows the experiment results. If the electrode did not break after 5 minutes, the process was described as a stable. Table 3 S/N Ratio for speed of cut and MRR Speed of cut MRR Experiment Process S/N ratio S/N ratio number status mm/min (mm2/min) [db] [db] 1 Stable ,522 2 Stable Unstable Stable Stable Stable Stable Stable Stable Unstable Unstable Stable Stable Unstable Stable Stable Stable Stable

4 19 Unstable Stable Stable Unstable Stable Stable Unstable Unstable Unstable For each measurement, the MRR values and cutting speed values were recorded, and the values of S/N ratio were calculated by [2]. where: n - number of repetition, yi - the measured value of the required characteristic. S/N = -10.log10[ 1 n y n i=1 i 2 ], [2] The next step was the use of analysis of variance (ANOVA) for the evaluation of the experiment. Analysis of variance (calculated from MRR) was performed to measure the significant and insignificant input parameters and also to clarify their impact on the monitored characteristics. It is necessary to determine whether the calculated S/N ratio has a normal probability distribution. Fig. 1 shows that the entire calculated S/N ratio meets the condition, and therefore it is possible to perform ANOVA. Fig. 1 Normal probability plot for S/N ratio 70

5 From ANOVA it was found (Table 4) that the input factors Time between two pulses (B), Injection pressure of the liquid dielectric (INJ), Wire tension (Wb) are irrelevant because the p- values are much greater than 0.05 (as it is the level of significance α = 0.05). Table 4 ANOVA for S/N ratio Source DF Seq SS Contribution Adj SS Adj MS F - value P - value A % B % Aj % INJ % Wb % FF % Residual Error % Total % It was found that the input parameters Pulse width (A), Servo reference mean voltage (Aj) and discharge frequency (FF) significantly influence the wire electrodes interruption, because its values are close to or smaller than p-value of 0.05 (significance level α). Parameter that significantly affects the interruption of wire electrodes is the discharge frequency (FF), because it is an input parameter with the highest net sum of squares (SEQ SSFF = ) and the lowest p-value 0.007, indicating a very strong influence on thinning wire electrodes. Table 5 Response table for S/N ratios Level A B Aj INJ Wb FF Delta Rank The response table for the signal to noise ratio includes the order of significance of factors based Delta Statistics that compare the relative size of each factor. The Statistics is the difference largest diameter and the smallest diameter of each factor. As can be seen from Table 5, the Discharge Frequency Parameter (FF) has the greatest impact on the wire electrodes breakage, which is also influenced by the pulse width (A) at the second place and the servo reference mean voltage (Aj) at the third place. CONCLUSION The following conclusions may be formulated from the experimental results: Increasing height of the workpiece will increase the probability of the wire electrode breakage; 71

6 Discharge frequency (FF), pulse width (A), Servo reference mean voltage (Aj) and dielectric fluid injection pressure (INJ) have a significant impact on the wire electrode breakage; The time between two pulses (B) and Wire tension (Wb) is statistically insignificant. It is probable that the interaction between the input parameters affecting the process of 30 %, as this is the value of the Residual error. Further research will examine the interaction between input parameters and their influence on the stability of the WEDM process. Acknowledgements The article was written within the VEGA 1/0477/14 project of Research of influence of selected characteristics of machining process on achieved quality of machined surface and problem free assembly using high technologies. References: 1. SHARMA, N., KHANNA, R., GUPTA, R., Multi Quality Characteristics of WEDM Process Parameters with RSM. Procedia Engineering ISSN SINGH, H., GARG, R., Effects of process parameters on material removal rate in WEDM. Journal of Achievements in Materials and Manufacturing Engineering, 32(1), pp ISSN SHAYAN, A. V., AFZA, R. A., TEIMOURI, R., Parametric study along with selection of optimal solutions in dry wire cut machining of cemented tungsten carbide (WC-Co). Journal of Manufacturing Processes, 15(4), pp ISSN RAMAKRISHNAN, R., KARUNAMOORTHY, L., Multi response optimization of wire EDM operations. The International Journal of Advanced Manufacturing Technology, 29(1-2), pp ISSN LIAO, Y. S., HUANG, J. T., SU, H. C., A study on the machining-parameters optimization of wire electrical discharge machining. Journal of Materials Processing Technology, ISSN S. R. NITHIN ARAVIND, S. SOWMYI, K. P. YUVARA, Optimization of metal removal Rate and surface roughness on Wire EDM using Taguchi method. In: IEEE- International Conference on Advances. In Engineering, Science and Management (ICAESM, 2012). ORCID: Vladimír Šimna